1. Technical Field
The present invention relates to a load control device and more specifically to a lighting dimmer that is operable when connected to either two or three active electrical wires.
2. Background Art
It is known to those skilled in the art, that standard 120 VAC residential wiring provides three conductors (e.g. wires) to which an installer can attach various equipment, such as lighting dimmers. It is also known that two of these three conductors are active current carrying conductors, meaning that connections to these active conductors are required to complete a functional circuit. The US National Electrical Code specifies, for 120 VAC residential wiring, that a first active conductor with significant voltage to earth ground be identified as a ‘hot’ (H) conductor and color-coded black and that a second active conductor near ground potential be identified as a ‘neutral’ (N) conductor and color-coded white. US National Electrical Code further specifies, for 120 VAC residential wiring, a third (passive) safety ground (G) conductor not normally used to carry circuit current and color-coded as green or as a bare uninsulated conductor.
Dimmers are devices used to vary the brightness of a lamp typically by modulating the duty cycle of the supplied AC power, such as by controlling a triac to turn on only after waiting for a period time has elapsed since a zero-crossing and allowing the triac to turn off at the next zero crossing. Although such phase control devices can be used for various purposes, the term dimmer is generally reserved for those intended to control lighting. By decreasing or increasing the AC duty cycle and hence the mean power to the lamp it is possible to vary the intensity of the light output from the lamp. Such a variable duty cycle AC voltage output is controlled by a dimmer, referenced to neutral (N), is identified as a ‘dimmer hot’ (DH) active conductor, and could be color-coded as blue (or red).
Lighting equipment manufacturers have developed two versions of residential lighting dimmers. A three-wire dimmer, known in the prior art, is shown in FIG. 1 and a two-wire dimmer, also known in the prior art, is shown in FIG. 2. As shown in FIG. 1, the three-wire dimmer 12 is connected such that its internal power supply is sourced directly from the hot (H) and neutral (N) line voltages of the AC voltage source 11. As shown in FIG. 2, the two-wire dimmer 22 is connected such that its internal power supply is sourced in series with the lighting load 13 to be dimmed, such as for example, an incandescent lamp. The two-wire dimmer 22 type is more prevalent in renovation work because it can be installed as a direct replacement for a wall switch without modifying the associated residential house wiring. The three-wire dimmer 12 type is typically used in new construction or where better performance is desired.
Refer now to FIG. 1. When a neutral (N) wire is present in a wall box, such as in new construction, it may be desirable to connect a three-wire dimmer 12 between the AC source 11 hot (H) and neutral (N) conductors and the lighting load 13. Such a neutral wire can be provided at the wall box by inserting a splice 14 into the circuit neutral wiring to the lighting load 13. Variable voltage, with respect to circuit neutral (N) is provided from the three-wire dimmer 12 as a dimmed hot (DH) output to the lighting load 13 hot (H) conductor. Advantageously, in this configuration a power supply, internal to the three-wire dimmer 12, will always be able to charge up through the hot (H) to neutral (N) path thus eliminating constraints on high end (maximum light output) and minimum load wattage. The neutral (N) wire connection also provides a means to obtain an accurate zero crossing signal from the AC source 11. The neutral (N) wire connection also allows for the three-wire dimmer 12 to operate as a full-on/full-off electronic switch since the output can go to full conduction. Such a full-on/full-off mode is desirable if the lighting load is replaced with a general appliance, such as an electric fan.
Refer now to FIG. 2, which shows common residential wiring practice when there no neutral (N) wire is present in the residential wall box. In this case, a two-wire dimmer 22 is serially connected between the AC source 11 hot (H) conductor and the lighting load 13 hot (H) conductor. The two-wire dimmer 22 hot (H) conductor connects to the AC power source 11 hot (H) conductor and the dimmer hot (DH) conductor from the two-wire dimmer 22 connects to the hot conductor (h) of the lighting load 13. In this two-wire configuration, the power supply within the two-wire dimmer 22 can only charge up when there is a voltage across the two-wire dimmer 22, for example, when a switching device internal to the two-wire dimmer 22, such as an internal triac, is not conducting.
The charging path for the two-wire dimmer 22 power supply is from AC Source 11 hot, through two-wire dimmer 22 exiting as dimmed hot (DH), through the lighting load 13, and then returning to AC source 11 neutral (N). Therefore, In order to charge up its internal power supply, the two-wire dimmer 22 internal switching device is required to be non-conducting for a portion of each AC half cycle. This limits the maximum lighting level of light load 13 and also limits the maximum load resistance that can be connected. Typically, connecting a lighting load 13 less than 50 watts at 120 VAC causes problems for a two-wire dimmer 22 power supply to charge up when at high end (e.g. maximum light level). This problem is exacerbated for two-wire dimmers that require additional current draw, such as those containing radio transceivers.
One of the specific problems in the field is that installers would like the choice of wiring dimmers in either the two-wire or three-wire configurations, as described above, but are desirous of only stocking one standard type dimmer to avoid the cost and complexity associated with stocking different part numbers.
In response to this problem, some manufacturers, such as Leviton Manufacturing Co., Inc., have developed a manually configurable three-wire dimmer product with a neutral (N) wire connection that can either be connected to AC source neutral (N) (for three-wire configuration) or externally connected to the lighting load along with the dimmed hot (DH) wire (for two-wire configuration). Such a configuration is illustrated in FIG. 3.
Refer now to FIG. 3. When operating in two-wire configuration, the neutral (N) wire on the manually configurable dimmer 32 needs to be connected to the lighting load 13 hot (H) wire along with the dimmed hot (DH) wire to ensure a symmetrical flow of current through the lighting load 13 when the power supply internal to the manually configurable dimmer 32, is charging. This is necessary especially for magnetic load types, such as low voltage transformers, because charging up asymmetrically can lead to heating in the transformer coil which can damage the transformer. Charging asymmetrically also limits the ability for the manually configurable dimmer 32 internal power supply to charge fully.
During installation, the manually configurable dimmer 32 can be set to operate in the above described two wire configuration by adding an external jumper 34 between the neutral (N) and dimmer hot (DH) terminals. The manually configurable dimmer 32 can alternatively be set to operate in the three-wire configuration by not including the external jumper 34 and wiring as described above for FIG. 1.
Typically, this additional external jumper 34 connection is simple to make as flying wire leads from the dimmer can be connected appropriately using twist-on wire connectors. However, it is now desirous for lighting dimmers to employ screw terminals similar to those found on standard light switches, such as a light switch being replaced by said dimmer, to make installation easier. In such an easier installation, the dimmer would be a drop-in replacement for the switch being removed and would not require bulky twist-on wire connectors to be squeezed into the residential wall box. These desired screw terminals pose a problem with making the external jumper 34 connection between the neutral (N) terminal and the dimmed hot (DH) terminal of the manually configurable dimmer 32 known in the prior art. The external jumper 34 would still be needed to configure the manually configurable dimmer 32 into a two-wire configuration as in the equivalent dimmer with flying leads but when screw terminals are introduced, this external jumper 34 jumper is likely to be ignored or misapplied by the installer resulting in faulty operation or failure of the device.
To solve the aforementioned problems associated with using a standard dimmer for both two-wire and three-wire dimming applications, the present invention is a unique simple and reliable standard dimmer adaptable to either two or three active wires.